1. Field of the Invention
The present invention relates generally to a wavelength division multiplexing optical transmission system, and more particularly to an optical transmitting device suitable for use in this transmission system.
2. Description of the Related Art
In a conventional long-distance optical transmission system traversing the ocean over a distance of thousands of kilometers, a plurality of regenerative repeaters for converting an optical signal into an electrical signal and performing retiming, reshaping, and regenerating are used to transmit the optical signal. At present, however, an optical amplifier has been progressively put into practical use, and an optical amplification repeating transmission system using the optical amplifier as a linear repeater is examined. The replacement of the regenerative repeater by the optical amplification repeater allows a great reduction in number of parts in the repeater, ensuring of reliability, and expectation of a great reduction in cost.
Further, as one of methods for realizing a large capacity of an optical transmission system, attention is focused on a wavelength division multiplexing (WDM) optical transmission system for multiplexing a plurality of optical signals different in wavelength in one optical transmission line and transmitting the multiplexed optical signals in the optical transmission line. In a WDM optical amplification repeating transmission system configured by combining the WDM optical transmission system and the optical amplification repeating transmission system, it is possible to collectively amplify a plurality of optical signals different in wavelength by using optical amplifiers, so that large-capacity and long-distance transmission of optical signals can be realized with a simple configuration.
As a conventional wavelength division multiplexing optical transmission system, there has been proposed a report on an optical amplification repeating transmission test employing eight channels, a transmission speed per channel of 5 Gb/s (total capacity of 40 Gb/s), and a transmission distance of 8000 km (circular length of 1000 km) (OFC'95, PD19, N. S. Bergano et al. AT&T). In the above report, the wavelengths of the eight optical signals are set at regular intervals of 0.53 nm in the range of 1556.0 nm to 1559.7 nm, and channel numbers are allocated to these different wavelengths from the shorter wavelength side. A 1.5 .mu.m zero-dispersion fiber (dispersion-shifted fiber, DSF) and a 1.3 .mu.m zero-dispersion fiber (high-dispersion fiber, HDF) are used as a transmission line.
The dispersion in the dispersion-shifted fiber is -2 ps/nm/km in average at a wavelength of 1558 nm. The dispersion in the high-dispersion fiber can be estimated to about 20 ps/nm/km. A circular loop is constructed of an optical transmission line and twenty-two optical amplification repeaters and one signal light level compensating optical amplifier inserted in the optical transmission line. The repeater spacing is 45 km. The dispersion-shifted fiber is used in the first repeating section to the twentieth repeating section, and the high-dispersion fiber is used in the twenty-first and twenty-second repeating sections. After circulating the circular loop eight times, each optical signal is subjected to dispersion compensation (post-compensation) by using a dispersion compensating fiber (DEF) on the receiving side. The length of the dispersion compensating fiber for each channel (signal light wavelength) is adjusted in performing the dispersion compensation.
In the above report, a bit error rate of 2.times.10.sup.-10 is reached, and there is almost no system margin. To enlarge a system margin, a method of increasing the output light power of the optical amplification repeater may be considered. In this case, however, since this method is largely affected by a non-linear effect of the transmission line, it is important to design the chromatic dispersion in the transmission line in sufficient consideration of the non-linear effect. FIG. 18 shows a chromatic dispersion map in the case where the signal light wavelength is 1558.0 nm. As apparent from FIG. 18, a chromatic dispersion of -1800 ps/nm occurs upon 900 km transmission, and this chromatic dispersion can be subsequently compensated up to almost 0 ps/nm by using the high-dispersion fiber having a total length of 90 km. Thereafter, this pattern is repeated.
FIG. 19 shows a chromatic dispersion map of the signal light having a wavelength of 1556.0 nm as the first channel. As apparent from FIG. 19, a chromatic dispersion of -1960 ps/nm occurs upon 900 km transmission, but a chromatic dispersion of about -160 ps/nm remains even after performing the dispersion compensation by the use of the high-dispersion fiber having a total length of 90 km. Accordingly, the final chromatic dispersion upon 8000 km transmission reaches -1280 ps/nm. FIG. 20 shows a chromatic dispersion map of the signal light having a wavelength of 1559.7 nm as the eighth channel. As apparent from FIG. 20, the final chromatic dispersion upon 8000 km transmission reaches +1100 ps/nm.
Accordingly, the difference between the remaining chromatic dispersion of the first channel and the remaining chromatic dispersion of the eighth channel becomes 2000 ps/nm or more. In the case where the output light power of the optical amplification repeater is small enough, an allowable chromatic dispersion quantity can be obtained in no consideration of the non-linear effect, and there is almost no problem because the allowable chromatic dispersion quantity is relatively large. However, in the case where the output light power of the optical amplification repeater is large, the allowable chromatic dispersion quantity is affected by the non-linear effect to become small. Therefore, there is a limit in the chromatic dispersion compensation (post-compensation) on the receiving side as described in the above report.
FIG. 21 shows the wavelength dependence of transmission characteristics in the case where only the post-compensation for chromatic dispersion is performed. In FIG. 21, a minimum eye aperture deterioration obtained by optimizing a post-compensation quantity is shown. As apparent from FIG. 21, a large eye aperture deterioration occurs in wavelength regions shorter and longer than a reference wavelength of 1558 nm. However, the output of the optical amplifier is assumed as +4 dBm.
In a long-distance wavelength division multiplexing optical amplification repeating transmission system, a chromatic dispersion quantity in each channel is largely slipped during transmission by the influence of chromatic dispersion slope (secondary dispersion) even when the chromatic dispersion in the transmission line is divisionally compensated, because of different signal light wavelengths in all channels. While the above report shows that the chromatic dispersion compensation (post-compensation) is performed by giving an optimal chromatic dispersion quantity to each channel on the receiving side, there is a limit in the extent of improvement as mentioned above.